The present invention relates generally to electric machines and, more particularly, to electric machines, such as interior permanent magnet machines and Synchronous Reluctance motor machines, that have offset rotor sections, thereby reducing torque oscillations and axial thrust in the rotor structure of the electric machine.
One general application for electric machines, and interior permanent magnet (IPM) machines in particular, is for use traction applications, such as when electric wheel motors (e.g., IPM) are connected to the wheels via a gearbox. This application typically requires very high torque at low speeds and yet maintenance of the rated power over a very wide speed range (e.g., on the order of 15:1).
A shortcoming of these types of motors is they are prone to high torque oscillations, or torque “ripple”. Further, depending on the magnitude of the torque “ripple” of the machine there may result damage to the rotor, the gearbox, and/or the mechanical system(s) connected to the IPM (due to fatigue or excessive torque). Additionally, the frequency of the torque ripple might excite resonant modes of the mechanical system(s), further posing an additional threat to the IPM and/or surrounding systems. With regards to torque ripple, similar attributes and shortcomings may also be found, in part, with Synchronous Reluctance motors.
There have been various attempts at reducing torque ripple. One trend is to construct the stator such that there are an odd number of stator slots per pole pair. While reducing torque ripple, this technique suffers from high core losses. Another technique for torque ripple reduction is stator skewing, wherein the stator is typically skewed in a single helicoidal path configuration. This technique, while aiding in the reduction of torque ripple, can add manufacturing complexity and costs while also introducing axial thrust to the stator assembly. Accordingly, there is an ongoing need for improving on current electric machine technologies and/or manufacturing thereof that address at least one of complexity, cost, efficiency, and/or performance without some of the current tradeoffs encountered with current methodologies.